The present invention relates to a scanning electron microscope for detecting a signal generated from a sample by irradiation of an electron beam and forming a scanned image of the sample.
A scanning electron microscope is an apparatus for scanning an electron beam which is generated from an electron source and finely limited by a focusing lens and an object lens on a sample using a deflector, detecting a signal generated from the sample by irradiation of the electron beam by a detector, and forming a sample image by processing the detection signal in synchronization with scanning of the electron beam. In order to improve the yielding rate of semiconductor devices, it is important to observe and analyze foreign substances and faults on devices. In correspondence with recent super refinement of semiconductor elements, observation and analysis of fine foreign substances and faults on semiconductor devices are required and for observation of foreign substances and faults, a scanning electron microscope is used instead of an optical microscope.
Information on irregularity such that observed foreign substances and faults are hollowed or projected is important information for analysis of foreign substances and faults. As a method for obtaining irregularity information of a sample, as described in Japanese Patent Publication 6-043885, there is a method for detecting a signal emitted in a direction at a small angle with the sample surface and forming a sample image. To observe fine foreign substances and faults, improvement of the resolution of a scanning electron microscope is desired. As a means for improvement of the resolution, by a method for bringing a sample close to an object lens or for leaking the magnetic field of an object lens on the sample side, the distance between the main surface of the object lens and the sample is made shorter.
According to the aforementioned prior art, a detector for detecting a signal emitted in a direction at a small angle with the sample surface must be arranged so as to look steadily at the electron beam irradiation position onto the sample and is inevitably arranged between the object lens and the sample. However, when the distance between the object lens and the sample is made shorter for improvement of the resolution, a problem arises that the amount of signals which can be detected by the detector is reduced and the SN ratio of sample images lowers. When an object lens of a magnetic field leakage type is used for improvement of the resolution, a problem arises that the track of a signal generated from a sample is bent by the magnetic field of the object lens and the signal cannot arrive at the detector.